Direction finding (DF) and Geolocation (GEO) approaches are commonly used to identify, track, and geolocate various sources of radio transmissions. DF is the process of obtaining the direction of arrival (DOA) bearings of radio signal emitter(s) of interest. On the other hand, GEO is the process of determining, either directly or indirectly based on DF estimates and/or other measures, the locations of radio signal emitter(s) of interest. DF and GEO techniques, which have been researched over the last few decades, are mostly understood. The theory and applications of DF/GEO are well described in the open literature and, as such, need not be further described herein. DF and GEO procedures for identifying and/or locating RF emitters are usually based on energy/amplitude comparison, interferometric, time-of-arrival (TOA), time-difference-of-arrival (TDOA), and other antenna null-steering approaches. These approaches usually demand special antennas, close-tolerance amplitude/phase RF receiver components, enhanced receiver dynamic range, and expanded processing bandwidth. DF/GEO systems can calculate the direction of arrival (DOA) of a particular RF emitter using an array of spatially displaced antennas or rotating antenna. Nearly all DF algorithms require that signals from multiple antennas are received and routed to multiple signal processors synchronously. These signal processors are then used to compare the amplitude/energy, phase, and TOA/TDOA from the various signals to derive the DOA and then location of the RF emitter.
One method for finding distance to an emitter is to use the received signal strength (RSS) or the received signal energy, which is an integration of the RSS over a pre-defined signal duration. Ignoring propagation channel irregularities, the RSS is inversely proportional to a function of the distance between the emitter and the receiver. However, when channel effects are included and depending on the amount of short-term fading variations the may be averaged out of the calculations, the root-mean square (RMS) path loss variations may possibly be up to ±10 dB. Because the processing algorithm is, in general, based on an indirect estimation of the RSS/energy at an RF sensor from the emitter, there are many potential errors. Potential errors occur because the RSS/energy not only depends on the transmit power, the distance between emitter and sensor receiver, but also depends on multi-path propagation, shadowing, and fading effects. In addition, in order to reduce estimation errors when working with the real data, the DF/GEO process must first remove the outlying samples or measurements by filtering or pre-screening the raw RSS/energy data, and/or averaging out several measurements prior to processing the data for DF/GEO calculations. Regardless, the energy-based method generally provides a more robust approach than other known methods when the emitter signal characteristics are unknown to an RF sensor, the observer.
In light of the foregoing, there is a need to find different or alternative energy-based approaches that can more quickly and more effectively DF and/or GEO RF emitters of interest.